Thanks again for all the input... looks, as always with FCL issues, like a 'can of worms'!
The situation is that I have the opportunity to fly said A/C as 'First Officer'. The Captain wants to operate always A/C as Multi-Crew but as A/C is privately operated as opposed to AOC this is where I have my concerns that I can get some form of formal/legal recognition with both FAA and JAR/EASA that I am 'actually' operating the A/C as opposed to just going along for the ride!
More research to be done on my part I think!....
CF